April 18 – Hot in here

Today’s Factismal: The first estimate of how Earth’s temperature would change with increased CO2 was made 118 years ago today.

There are very few scientific discoveries that are as misunderstood by the public or as unfortunately politicized as the effects of carbon dioxide on temperature. Though the basic effect was first described in 1859, it wasn’t until 1896 that Svante Arrhenius realized that changing the amount of carbon dioxide in the atmosphere could change the global temperature. And it wasn’t until a century later that the results of their work would become politicized.

The way that CO2 works has nothing to do with how greenhouses work; unfortunately, because Svante used a greenhouse as an analogy in his paper, we are stuck with calling it “the greenhouse effect”. (The lesson here is that you should always be careful with your analogies.) But in order to understand the greenhouse effect, you need to understand both heat and light. Fortunately, the two are related.

The hotter the object, the shorter the wavelength of light it emits (Image courtesy LED.com)

The hotter the object, the shorter the wavelength of light it emits
(Image courtesy LightEmittingDiodes.org)

We know that light isn’t limited to what we can see. Instead, it spreads across a wide spectrum that extends from photons with kilometer lengths (radio waves) through light with centimeter wavelengths (microwaves) and light with micrometer wavelengths (infrared light) to light with angstrom wavelengths (visible light) and light with wavelengths shorter than a molecule (ultraviolet light) and even light with wavelengths small enough to fit inside an atom (X-rays). But what is truly nifty about the wavelength of light is that it is a direct measurement of the amount of energy it has; the shorter the wavelength, the more energy.

So ultraviolet light is ultra-violent when compared to visible light which itself is more energetic than infrared (in-da-bed) light. And only things that are very hot (which is the same as saying very energetic) can make photons that are energetic. Cool things make photons with long wavelengths and hot things make photons with short wavelengths.

Now let’s apply this to the Earth. When sunlight, which is rich in visible and ultraviolet light because of the Sun’s temperature, shines on a planet, the planet absorbs the light and heats up. As the planet’s temperature goes up, it starts to emit light. But, because the planet is cooler than the Sun, it gives off light at longer wavelengths. The visible and ultraviolet light of the Sun is transformed into infrared light. We can calculate just how hot the planet should be using an ideal case known as a blackbody and get a blackbody temperature. (Sound complicated? It is as easy as using an ear thermometer, which is based on the same idea.) But the Earth, Venus, and Mars are all hotter than they should be.

The increase in global temperature is driven in large part by the increase in CO2 (Image courtesy NOAA)

The increase in global temperature is driven in large part by the increase in CO2
(Image courtesy NOAA)

The reason for that is because of CO2. CO2 happens to be opaque in IR; that is, it blocks some of the “heat radiation” given off by the Earth. This is reabsorbed by the atmosphere, raising its temperature slightly. Of course, lots of other factors come into play when you are talking about a planet , so the temperature change isn’t instantaneous and it has some wiggles in it. But overall, the pattern is clear: increasing CO2 increases temperature and changes climate.

If you’d like to learn more about the effects of CO2 and what you can do to help, then why not look at the MIT Climate CoLab?


4 thoughts on “April 18 – Hot in here

  1. Pingback: August 2 – What a gas! – Little facts about science

  2. Pingback: June 24 – On Wings Of Eagles | Little facts about science

  3. Pingback: September 21 – The Heat Is On | Little facts about science

  4. Pingback: January 18 – Children Of The Sun | Little facts about science

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